Influence of Different Cooking Methods on Physicochemical ...curresweb.com/mejas/mejas/2017/1127-1147.pdf · Received: 18 Sept. 2017 / Accepted: 22 Nov. 2017 / Publication date: 30

Middle East Journal of Applied Sciences ISSN 2077-4613

Volume : 07 | Issue :04 |Oct.-Dec.| 2017 Pages: 1127-1147

Corresponding Author: Mohsen M. S. Ashour, Food Science and Technology Department, National Research Centre, Dokki, Egypt. E- mail: [email protected]

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Influence of Different Cooking Methods on Physicochemical Characteristics, Phytochemical Profile, Antioxidant Capacity and Chromatic Parameters of

Selected Vegetables

Mohsen M. S. Ashour and Enssaf M. A. El-Hamzy

Food Science & Technology Department, National Research Centre, Dokki, Cairo, Egypt

Received: 18 Sept. 2017 / Accepted: 22 Nov. 2017 / Publication date: 30 Dec. 2017

ABSTRACT

The objective of the present study was to evaluate the effect of three most common Egyptian cooking practices (i.e., boiling, steaming, and frying) on physicochemical characteristics (i.e., shear force, percent softening and chromatic parameters), phytochemical profiles (i.e., polyphenols, carotenoids, ascorbic acid, and glucosinolates), and total antioxidant capacities (TAC), as measured by three different analytical assays [Trolox equivalent antioxidant capacity (TEAC), total radical-trapping antioxidant parameter (TRAP), ferric reducing antioxidant power (FRAP)] of three selected vegetables (Carrots, Zucchini and Broccoli). Boiled cooking treatments better preserved the antioxidant compounds, particularly carotenoids, in all vegetables analyzed and ascorbic acid in carrots and zucchini. Steamed vegetables maintained a better texture quality than boiled ones, whereas boiled vegetables showed limited discoloration. Fried vegetables showed the lowest degree of softening, even though antioxidant compounds were less retained. An overall increase of TAC values (TEAC, FRAP, and TRAP) was observed in all cooked vegetables, probably because of matrix softening and increased extractability of compounds, which could be partially converted into more antioxidant chemical species. In this investigation, the three vegetables did not demonstrate a consistent trend under the same cooking conditions, which could be ascribed to morphological features, specific components, and antioxidants profile of vegetables. Our findings defy the notion that processed vegetables offer lower nutritional quality and also suggest that for each vegetable a cooking method would be preferred to preserve or improve its physicochemical characteristics and antioxidant properties. This selection may help consumers on the choice of cooking practices to improve the nutritional quality of foods, as well as their acceptability.

Key words: Cooking Methods; Phytochemical Profile; Antioxidant Capacity; Texture; Shear force;

Color Parameters; Carrots; Zucchini (green squash); Broccoli.

Introduction

The nutritional quality provided by vegetables and fruits consumption has been intensely reviewed (He et al., 2007; Tiwari and Cummins, 2013). Vegetables are rich sources of proteins, fats, carbohydrates, minerals, antioxidants, fiber and water, as well as being excellent sources of β-carotene (provitamin A), thiamin (B1), riboflavin (B2), niacin, pyridoxine (B6), pantothenic acid, folic acid (folacin), ascorbic acid, and vitamin E and K (Prodanov et al., 2004). Vegetables and fruits are considered particularly protective thanks to their content of phytochemicals. These naturally occurring compounds have attracted great attention from the scientific community for their antioxidant properties and their implication in a variety of biological mechanisms at the base of degenerative processes (Kaur and Kapoor, 2001). Such compounds are secondary plant metabolites responsible for plant food color, smell, flavor, and bitterness and consist of a wide variety of different molecules, such as carotenoids, polyphenol compounds, vitamins, and glucosinolates. Most vegetables are commonly cooked before being consumed. It is known that cooking induces significant changes in chemical composition, influencing the concentration and bioavailability of bioactive compounds in vegetables (Bongoni et al., 2014; Kahlon et al., 2012).

Physical properties of vegetables are also greatly affected by heat treatments (Turkmen et al., 2006). Texture and color are considered very important parameters in the cooking quality of

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vegetables, and they may strongly influence consumer purchases of these food items. Changes in texture are often dramatic because of the membrane disruption and the associated loss of turgor (Tiwari and Cummins, 2013). In addition, cooked vegetables exhibit poor color quality in comparison with fresh ones (Turkmen et al., 2006). However, both positive and negative effects have been reported depending upon differences in process conditions and morphological and nutritional characteristics of vegetable species (Podsêdek, 2007; Poelman et al., 2013).

Recent studies have shown there are several ways to enhance the availability of healthy nutrients through proper selection of the method of cooking. According to these studies, the most common methods used for cooking vegetables are: steaming, roasting, boiling, frying, sautéing, sous vide, microwave and pressure-cooking. Besides that, the authors also considered in their researches, factors related to common domestic processing, including: washing, peeling, cutting, chopping and soaking (Tiwari and Cummins, 2013). Such information has been studied for specific vegetables such as: broccoli and carrots (Dos-Reis et al., 2015; Murador et al., 2014).

Many reports have found significant differences among the cooking methods. Kahlon et al. (2007) studied how cooking could influence in vitro bile acid binding by various vegetables. It has been demonstrated that bile acid binding lowers the levels of cholesterol in the blood, helping to reduce the risk of heart disease (Tiwari and Cummins, 2013). In their first study they found that steam cooking improved bile acid binding by beets, eggplant, asparagus, carrots, green beans and cauliflower when compared to the same vegetables uncooked. In their next study, the authors obtained similar results by steaming collard greens, kale, mustard greens, broccoli, Brussel sprouts, spinach, green bell pepper and cabbage (Adriana et al., 2016; Kahlon et al., 2008). After four years, the authors studied some of the same vegetables of the second study using other cooking methods (sautéing, boiling, steaming). They concluded that sautéing was the cooking method with the most health potential (binding bile acids) for mustard greens, kale, broccoli, cabbage and green bell pepper, with steaming the best method used for collard greens (Kahlon et al., 2012).

Since the early part of the twentieth century many studies have been conducted to investigate the impact of preparation and cooking methods on the stability of nutrient sin food. The results of these studies vary widely leading the consumer to question the best ways of preparing and cooking foods in order to maintain the nutritional qualities, especially in legumes and vegetables. Many other researchers have shown that growth conditions of vegetables and legumes also have a significant impact on their nutrient content (Elmore et al., 2010; Lee et al., 2009; Wang et al., 2010).

Vegetables have also been associated as part of a healthy diet, by reducing the risk of some chronic diseases (Tiwari and Cummins, 2013). Vegetables provide vital nutrients for healthiness and maintenance of the human body, such as vitamin A, vitamin C, folate, fiber and potassium. Vegetables as broccoli, provide flavonoids (Lin and Chang, 2005), polyphenols (Faller and Fialho, 2009), anthocyanin with high antioxidant activity (Monero et al., 2010) and powerful phytochemicals (glucosinolates and isothiocyanates). Isothiocyanates and glucosinolates are the main biologically active compounds that are known to exhibit anti-carcinogenic activity in several in vitro and in vivo studies (Moreno et al., 2006; Verkerk et al., 2009). According to the WHO (2003), the classification of vegetables can vary from country to country. The large reason for this differentiation is related to the inclusion or exclusion of starchy roots, tubers and legumes, within the vegetable groups.

Although consumption of fresh unprocessed plant food is widely advocated, evidence is emerging that in vivo bioavailability of many protective compounds is enhanced when vegetables are cooked (Link and Potter, 2004). However, data on the effect of cooking on nutritional properties of vegetables are still incomplete (Link and Potter, 2004). In fact, literature data on nutritional properties of cooked vegetables often deal with a single vegetable (Glyszczynska-S� wiglo et al., 2006), a family of vegetables (Podsêdek, 2007), or a single phytochemical group (Lee and Kader, 2000). A more integrated analysis of nutritional and physical properties of vegetables is needed to obtain insight into the effect of cooking.

Therefore, the purpose of this study was to evaluate the effect of three common Egyptian cooking practices (i.e., boiling, steaming, and frying) on physicochemical characteristics (i.e., shear force, softening (%) and chromatic parameters), phytochemical profiles (i.e., polyphenols, carotenoids, ascorbic acid, and glucosinolates), and total antioxidant capacities (TAC), as measured by three different analytical assays [(TEAC), (TRAP), and (FRAP)] of three selected vegetables (i.e., Carrots, Zucchini and Broccoli), chosen on the basis of their different morphological feature,


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nutritional profiles, and antioxidant capacities (Pellegrini et al., 2003; and 2010). Nutritional data are also compared to changes of texture and color induced by cooking.

Materials and Methods Materials 1. Samples

Freshly harvested carrots (Daucus carota L.), zucchini or green squash (Cucurbita pepo L.), and broccoli (Brassica oleracea, var. botrytis caput L.) of a single batch were purchased from a local market. The selected vegetables were stored at 4±1°C before processing for a maximum time period of three days. The samples were selected visually by color, size and freshness, and with no sign of mechanical damage.

2. Chemicals.

All reagents and solvents used were HPLC-grade and purchased from Merck (Germany). High-purity water was produced in the laboratory using an Alpha-Q system (Millipore, Marlborough, MA).

The 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), 2,4,6-tripyridyl-s-triazine (TPTZ), β-carotene, lutein, quercetin, rutin, chlorogenic acid, caffeic acid, p-coumaric acid, ferulic acid, sinapic acid, kaempferol, morin, 2,6-di-tert-buthyl p-cresol (BHT), sinigrin (allyl glucosinolate) and 2,2′-azobis (2-amidinopropane) dihydrochloride (ABAP) were purchased from Sigma-Aldrich (Sigma Chemical Co., St. Louis, USA). R-phycoerythrin (R-PE) was purchased from Prozyme (San Leandro, CA).

Methods 1. Preparation of Selected Vegetables:

Briefly, the fresh samples were stored at 4 ± 1°C and immediately analyzed for physicochemical and quality parameters. The fresh carrots, peeled before processing, and zucchini (green squash) were prepared by cutting off the top and bottom ends with a knife and extracting cylindrical specimens (diameter, 25 mm; height, 25 mm) from each sample. Broccoli were cleaned by removing the inedible parts and then chopped into homogeneous pieces, leaving a stem of 25 mm.

To obtain more homogeneous samples, each vegetable was prepared in batches of 1 Kg. Each batch was then divided into four equal portions. One portion (250 g) was retained raw (fresh), and the others were cooked in three different methods in triplicate, as given below.

2. Cooking Treatments:

Three of the most common cooking methods used by the Egyptian population (i.e., boiling, steaming, and frying) were used. Cooking conditions were optimized by preliminary experiments carried out for each vegetable. For all cooking treatments, the minimum cooking time to reach a similar tenderness for an adequate palatability and taste, according to the Egyptian eating habits, was used.

2.1. Boiling:

Vegetable material was added to boiling tap water in a covered stainless-steel pot (1:5 food/water) and cooked on a moderate flame. For each cooking trial, 10 samples were boiled. Then, samples were drained off for 30 s. After boiling, all the samples were cooled rapidly on ice.


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2.2. Steaming: Steaming treatments were carried out in a Combi-Steal SL oven (V-Zug, Zurich, Switzerland). Nine specimens were placed in the oven equilibrated to room temperature before each cooking trial. Eight samples were arranged in a circle, and one was placed at the center to ensure uniform heating conditions in all samples for each cooking trial. The samples were cooked under atmospheric pressure. 2.3. Frying:

Vegetable was added to 2.2 L of corn oil in a domestic deep fryer (DeLonghi, Italy) at 170 °C. A total of 10 samples were fried for each cooking trial. At the end of each trial, samples were drained off and dabbed with blotting paper to allow for the absorption of exceeding oil.

After all cooking experiments, samples were cooled rapidly on ice, packed in polyethylene bags with nitrogen gas added and kept at 4 ± 1°C. The determination of TAC and antioxidant compounds was analyzed within 24 hours after cooking. The texture analyses were performed on cooked samples at 50°C, referred as the temperature of consumption, while color analyses were performed at room temperature (25°C). Both temperatures were controlled inserting a thermocouple (K-type; Ni/Al-Ni/Cr) connected to a multimeter acquisition system (Keithley Instruments, Inc., Cleveland, OH) to the thermal center of one sample for each cooking trial.

3. Analytical Methods 3.1. Dry Matter Determination:

For the determination of the moisture, 3–4 g of raw or cooked homogenized sample (as triplicate) was dried in a convection oven at 105°C for at least 16 h until reaching a constant weight.

3.2. Texture Analysis:

Texture of the raw and cooked samples was analyzed by a shear force test using a TA.XT2 Texture Analyzer equipped with a 25 kg load cell (Stable Micro Systems, Goldalming, U.K.), and the parameters were quantified using the application software provided (Texture Expert for Windows, version 1.22).

3.2.1. Shear Force Analysis:

Shear force analysis was performed using a Warner-Bratzler blade (3 mm thick), which cut the cylindrical specimen of carrot and zucchini (green squash) cylinders parallel to their major axis, whereas broccoli was cut between the flower heads and the end of the stems at a constant speed of 60 mm/min and pushed through the slot (4 mm wide). The maximum force (N) required to shear the sample was measured. A total of 15 determinations were performed for each cooking treatment.

3.2.2. Percent Softening:

Percent softening was calculated as:

Softening (%) = (1 - ��

�� ) � 100 ……………………………………. (1)

3.3. Color Analysis:

Color determination was carried out using a Minolta Colorimeter (Hunter Lab, model MiniScan�� XE Plus, Reston, VA, USA). Both raw and cooked samples were analyzed. The assessments were carried out both on the external and internal surfaces of the cylindrical specimens of carrots and zucchini (green squash) cut parallel to their major axis and on the florets and stems of


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broccoli. Color was expressed in CIE L* (lightness; black = 0, white = 100), a* (redness > 0, greenness < 0), b* (yellowness > 0, blue < 0) coordinates, standard illuminant D�� and observer 10° (Ashour and Shaaban 2014; El-Hamzy et al., 20016). Fifteen replicate measurements were performed and results were averaged for each cooking treatment. In addition, color intensity C* (Chroma, 0 at the centre of the color sphere), and H° (hue angle, red = 0°, yellow = 90°, green = 180°, blue = 270°) were calculated using the following Equations (2) and (3), where: �� and �� are the control values for samples (Sigge et al., 2001).

Chroma (C*) = (�∗� + �∗�)�.� ……………………………………………………………....... (2) Hue angle (HO) = �� (�∗ �∗⁄ ) ……………………………………………………………....... (3) 3.4. Determination of Antioxidant Compounds and Total Antioxidant Capacity (TAC):

For the analyses of antioxidant compounds, with the exception of ascorbic acid, the samples were freeze-dried using a laboratory freeze dryer (Freeze Mobile 24, Virtis Company, Inc., Gardiner, NY). Dried sample material was finely ground, kept in sealed bags, and stored at -20°C. Analyses of ascorbic acid and TAC were performed on fresh samples within 24 h of cooking.

For all of the parameters considered (e.g., TAC and carotenoid and polyphenol content), the cooking effect was reported as the percent variation with respect to uncooked vegetable and calculated according to the following equation:

Variation (%) = ( ��

�� ) � 100…………………………...... (4)

3.4.1. Determination of Carotenoids:

The determination of carotenoids was carried out by high-performance liquid chromatography (HPLC) analysis as previously described by Leonardi et al., (2000). Briefly, 0.1 g of lyophilized sample was extracted with tetrahydrofuran containing 0.01% BHT as the antioxidant agent, dried under nitrogen flow in dark tubes, resuspended in dicloromethane, and analyzed using a HPLC (Shimadzu LC10, Japan) controlled by Class VP software (Shimadzu, Japan) with a diode array detector (SPD-M10A Shimadzu, Japan) and a Prodigy column (5 μm ODS3 100A, 250 × 4.6 mm; Phenomenex, Torrance, CA). The injection volume was 20 μL, and the carotenoids were eluted with a flow of 0.8 mL/min, following this linear gradient: starting condition, 82% A and 18% B; at 20 min, 76% A and 24% B; at 30 min, 58% A and 42% B; at 40 min, 40% A and 60% B; and at 45 min, 82% A and 18% B. Phase A was a mixture of acetonitrile, n-hexane, methanol, and dichloromethane (2:1:1:1, v/v/v/v), while phase B was acetonitrile. Identification of the peaks in the HPLC chromatogram of the carotenoid extract was carried out by a comparison of UV–vis spectra or with retention times of eluted compounds with pure standards at 450 nm for α- and β-carotene, β-cryptoxanthin, and lutein; at 350 nm for phytofluene and at 290 nm for phytoene. To quantify phytofluene, phytoene, β-cryptoxanthin, and α-carotene, their respective peak areas were compared to the ones of standard β-carotene at known concentrations, established by the molar extinction coefficient in acetone reported in the literature and corrected by the molar extinction coefficient relative at each compound. The identification of cis-carotene isomers was based on spectral characteristics as described by Hu et al. (2008); Kao et al., (2012). Because no standards for cis isomers are available, the quantification was carried out using the calibration curve of all trans isomers.

3.4.2. Determination of Polyphenols:

Polyphenols before and after deglycosilation, were determined following the procedure described by Juániz et al. (2016), with few modifications. Briefly, 1 g of lyophilized sample was extracted with 10 mL of 60% aqueous methanol solution containing 0.25 mg of morin as an internal standard. About 1.5 mL of this solution was kept down; the remaining part was hydrolyzed by adding with 20 mM sodium diethyl-dithiocarbamate and 5 mL of 6 M HCl, and then it was refluxed at 90 °C


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for 2 h. A total of 20 μL of the extract, taken both before and after hydrolysis, was analyzed by HPLC (Shimadzu LC 10, Shimadzu, Japan) with a diode array detector and a Prodigy column (5 μm ODS3 100A, 250 × 4.60 mm; Phenomenex, Torrance, CA) at a flow rate of 1 mL/min. The mobile phase was a mixture of water/formic acid (95:5, v/v) (A) and methanol (B). Flavonoid elution was achieved using the following linear gradient: starting condition, 70% A and 30% B; at 3 min, 50% A and 50% B; at 18 min, 40% A and 60% B; at 23 min, 20% A and 80% B; at 28 min, 10% A and 90% B; and at 33 min, 70% A and 30% B. Chromatograms were recorded at 256 nm for flavonols and at 325 nm for phenolic acids.

3.4.3. Determination of Ascorbic Acid (AA):

Ascorbic acid (AA) was determined based upon the quantitative discoloration of 2,6-dichlorophenol indophenol titrimetric method as described in AOAC methodology No. 967.21 (AOAC, 2000). Comparative evaluations of Ascorbic acid stability in raw (fresh) and cooked samples by different methods were carried out, where 5.0 ± 0.1 g of each sample was weighed, crushed and diluted in 1 L distilled water. The Ascorbic acid content was expressed as mg AA retained/100 g dry matter.

3.4.4. Evaluation of Total Antioxidant Activity (TAC): 3.4.4.1. Preparation of vegetable extracts

Extractions were carried out as previously described by Ferracane et al. (2008), with a few modifications. The extraction procedures were carried out under dim light to prevent photodegradation. Ten grams each of raw or cooked samples by different methods in a centrifuge tube (50 mL) with a stopper were homogenized under nitrogen flow for 1 min., with 40 mL 60% ethanol, in a homogenizer (Model 01-01200; PRO Scientific, Oxford, CT, USA) and centrifuged at 1000g for 5 min., and the supernatant was collected. The precipitate was re-extracted by adding 20 mL 60% ethanol, homogenizing for a further 1 min., and centrifuged at 1000g for 5 min. This ethanol extraction was repeated four times, and the resulting supernatants were combined and dried under vacuum, at a temperature below 30°C. The residue was then redissolved by ultrasonic agitation to a final volume of 20 mL in 60% ethanol, and this ethanol extract was used for the following analyses.

3.4.4.2. Determination of TAC:

The TAC values were determined as previously described in Pellegrini et al. (2003). Briefly, raw and cooked samples were homogenized under nitrogen flow in a high-speed blender (Brawn Multimix MX32). A weighed amount (∼1 g) was extracted with 4 mL of water under agitation for 15 min at room temperature and centrifuged at 1000g for 10 min, and the supernatant was collected. The extraction was repeated with 2 mL of water, and the two supernatants were combined. The pulp residue was re-extracted by the addition of 4 mL of acetone under agitation for 15 min at room temperature and centrifuged at 1000g for 10 min, and the supernatant was collected. The extraction was repeated with 2 mL of acetone, and the two supernatants were combined. All food extracts were adequately diluted in the appropriate solvent (depending upon their activity) and immediately analyzed in triplicate for their antioxidant capacity by three different TAC assays: Trolox equivalent antioxidant capacity (TEAC) assay (Scalzo et al., 2005), Total radical-trapping antioxidant parameter (TRAP) assay (Ferracane et al., 2008), and Ferric reducing antioxidant power (FRAP) assay (Ou et al., 2002). The TEAC and TRAP values were expressed as millimoles of Trolox per 100 g of sample. FRAP values were expressed as millimoles of Fe+2 equivalents per 100 g of sample.

3.5. Glucosinolates:

The determination of glucosinolates was carried out after the desulphation reaction according to the procedure described by Kiddle et al. (2001), with some modifications. Briefly, 0.2 g of freeze dried of raw or cooked broccoli by different methods was extracted with 3.5 mL aqueous methanol


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(70:30, v/v) and heated at 70 °C in a heating bath for 10 min. The extracts were centrifuged at 2000g for 10 min at 4 °C; the supernatant was refrigerated, while the pellet was extracted a second time with 3 mL of aqueous methanol (70:30, v/v), heated at 70 °C, and centrifuged using the previous conditions. The two supernatants were combined and refrigerated. The desulphation reaction was performed with minicolumns prepared with 1 mL of Sephadex A25 and 2 M acetic acid (1:1, w/v) to have a 0.5 mL bed volume. Columns were washed with 6 M imidazole formate and with ultrapure water, and then 1 mL of the glucosinolate extract was added. The unbound material was removed washing with 0.1 M sodium acetate (pH 4); then 100 μL sulfatase (EC 3.1.6.1) was loaded in the column, and desulphation was performed overnight (16 h) at room temperature. The desulphoglucosinolates were eluted with 1.5 mL of ultrapure water and stored at -20 °C before analysis. A total of 20 μL of the extract was analyzed by HPLC (Shimadzu LC 10, Shimadzu, Japan) at a flow rate of 1 mL/min, using a Prodigy column (5 μm ODS3 100A, 250 × 4.60 mm; Phenomenex, Torrance, CA). Desulpho-glucosinolates elution was achieved using the following linear gradient: starting condition, 2% B; at 5 min, 4% B; at 20 min, 20% B; at 30 min, 35% B; at 35 min, 40% B; at 45 min, 30% B; at 50 min, 10% B; and at 52 min, 2% B. The mobile phases were water (A) and methanol (B). Chromatograms were recorded at 227 nm. Sinigrin was used as an internal standard. The confirmation of the compound identity was achieved by HPLC MS-MS, as recently described (Mølmann et al., 2015).

4. Statistical Analysis:

The effect of different cooking methods on each quality parameter was estimated using Sigma-Plus 13 (Statistical Graphics Corp., Herndon, VA, USA). The results were analyzed by an analysis of variance (ANOVA). Differences amongst the media were analyzed using the least significant difference (LSD) test with a significance level of α = 0.05 and a confidence interval of 95% (p ≤ 0.05). In addition, the multiple range test (MRT) included in the statistical program was used to demonstrate the existence of homogeneous groups within each of the parameters.

Results and Discussion Carrots: 1. Effect of Cooking on Physicochemical Characteristics:

Figure 1 shows shear force values obtained for raw and cooked carrots by three different cooking methods. The degree of softening induced by the different cooking treatments as referred to the raw samples was also reported in the same graph. Raw carrots showed a shear force value of 153.1 ± 4.1 N. Cooking of carrots caused a decrease in the force needed to shred the vegetable (Figure 1), indicating a decrease of firmness and consequently softening of the vegetable for all of the three cooking methods. In particular, boiled carrots showed a significantly lower shear force value (higher degree of softening, > 96.0%) in comparison to both steamed and fried samples (92.0% and 89.0%, respectively).

The color change of a food product during cooking is an indication of how severe cooking conditions are related to its pigment composition/concentration. Different cooking methods and pretreatments exert a significant effect on the color changes of carrots slices. Table 1 shows the color data in terms of L*, a*, b*, C*, and H° values of raw (fresh) and cooked carrot slices.

For carrots samples, both external and internal surfaces were considered. The color of the internal surface of raw carrots had coordinate L* of 62.4 ± 2.3 (lightness), coordinate a* of 32.9 ± 0.7 (redness), and coordinate b* of 42.5 ± 1.9 (yellowness). Coordinates L*, a*, and b* values significantly decreased after all cooking treatments. The external color of the cooked samples was less bright (L*), red (a*), and yellow (b*) than the raw sample color. A significant loss of vivid color (C* decrease) with respect to raw carrots was observed for the external surface of all cooked samples, whereas a significant C* increase was noticed internally. The hue angle significantly increased both for the external and internal surfaces of cooked carrot samples by boiling method in comparison to the


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raw samples, resulting in a shift from red to orange. In the case of steamed and fried carrots, the hue angle also significantly increased but only on the external surfaces.

The saturation index or chroma (C*) and the hue angle (HO), as shown in Table 1, provide more information about the spatial distribution of colors than direct values of tristimulus measurements (Abonyi et al., 2002; Rico et al., 2010). It is observed that the values of both indices are affected by cooking method in opposite ways (p > 0.05) for both fresh and cooked carrot slices. Estimated chroma values (C*) of the cooked carrot samples retained the 84.22% and 81.76% of the cooked samples by (boiling and steaming) methods, respectively, the hue angle (HO) showed an increase of 27.90% and 16.90% in the same samples, respectively compared to fresh sample indicating discoloration of the external surface carrot slices color. The HO values of the external surface carrot slices were in the range of 50.9 ± 0.9 to 65.1 ± 1.1, which were significantly (P≤0.05) different from samples obtained with different cooking methods.

Table 1: Effect of different cooking methods on the chromatic parameters (L*, a*, b*, C*, and H° values) of raw and cooked Carrots۞.

Cooking Methods

Chromatic Color

L* a* b* C* H° Internal surface Raw 62.4 ± 2.3a 32.9 ± 0.7a 42.5 ± 1.9a 34.9 ± 1.6b 69.1±2.1ab Boiled 47.1 ± 1.3c 12.8 ± 0.9c 38.7 ± 1.7b 39.5 ± 1.4ab 72.2 ± 1.6a Steamed 52.1 ± 1.9b 16.6 ± 1.6b 38.9 ± 1.9b 41.4 ± 2.3a 68.9±2.1ab Fried 50.3 ± 0.9b 18.1 ± 1.7b 38.5 ± 2.0b 42.2 ± 1.9a 66.8 ± 1.5b External surface Raw 57.4 ± 1.7a 31.3 ± 1.7a 37.1 ± 2.1a 48.8 ± 2.4a 50.9 ± 0.9c Boiled 53.6 ± 1.5b 16.9 ± 1.8c 36.6 ±2.2ab 41.1 ± 2.7b 65.1 ± 1.1a Steamed 51.3 ± 1.1c 21.0 ± 1.2b 34.1 ± 1.6bc 39.9 ± 1.6b 59.5 ± 1.5b Fried 41.4 ± 2.1d 21.9 ± 1.5b 31.9 ± 2.4c 38.9 ± 2.6b 56.2 ± 1.2b ۞ Values are expressed in colorimetric units and presented as mean ± SD (n = 15). Different letters in the same column indicate that the

values are significantly different (p ≤ 0.05).

The color of carrots has been reported to be largely due to the presence of carotenes, which is in turn deeply affected by variety, maturity, and growing conditions (Koç et al., 2017). Thus, a comparison of color parameters obtained in this study for raw and cooked carrots with data reported in the literature is difficult, because of the high variability of these vegetables.

The α- and β-carotene content was reported to influence the color of these vegetables (Jorge et al., 2017). Although such carotenes are known to be relatively heat-stable (Koç et al., 2017), they isomerize into various cis isomers during cooking (Aman et al., 2005). The larger L*, a*, b*, and C* decrease observed in all cooked carrots may be related to the α- and β-carotene decrease and their


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isomerization, as already observed for heat-processed carrot juice (Kao et al., 2014). In addition, the hue angle (HO) increase could be related to the decrease of the carotene amount, as observed by Sulaeman et al. (2004), who reported a high negative correlation between this color parameter and the carotene content of deep fried carrots. The outer part of carrots was also reported to contain twice as much β-carotene as the inner part (Kao et al., 2012), and this may explain the remarkable loss of chroma (C*) and the shift of the hue angle (H°) to yellow observed externally.

2. Effect of Cooking on the Phytochemical Profile:

The effects of different cooking methods of cooked carrots on Carotenoids, Polyphenols, and Ascorbic acid are reported on a dry weight basis in Table 2, in comparison to the antioxidant concentrations measured in raw samples.

Raw carrots showed high concentrations of two vitamin A precursors, the α- and β-carotene (4.46 and 6.42 mg/100 g of fresh weight corresponding to 38.1 and 54.9 mg/100 g on a dry weight basis).

Cooking had a small but significant effect on total carotenoids (p ≤ 0.05): boiling determined a slight increase of 15.1% of their initial concentration, while the other two methods (steaming and frying) caused a slight but significant decrease, more evident in the case of frying (-12.2%). Among single carotenoids, lutein was slightly increased by boiling (+6.6%), whereas 37.2% and 46.3% of its initial concentration was lost during steaming and frying, respectively. α-Carotene decreased significantly after all cooking methods, even though its retention was lower after steaming and frying. β-Carotene was not significantly influenced by boiling (p ≤ 0.05), but its concentration decreased slightly but significantly during steaming (-9.5%) and frying (-22.8%).

Table 2: Effect of different cooking methods on the antioxidant compounds of raw and cooked Carrots۞. Antioxidant Compounds Cooking Methods

Raw Boiled Steamed Fried Carotenoids

Llutein 12.1 ± 0.4a 12.9 ± 0.2a 7.6 ± 0.2b 6.5 ± 0.1b α-Carotene 38.1 ± 0.5a 31.9 ± 0.4b 28.4 ± 0.2c 25.4 ± 0.3d β-Carotene 54.9 ± 0.4a 56.3 ± 0.6a 49.7 ± 0.7b 42.4 ± 0.8c cis-Carotene ND 18.8 ± 0.4 11.6 ± 0.2 10.9 ± 0.2 Phytoene 6.8 ± 0.8d 8.4 ± 0.2b 7.6 ± 0.6bc 10.1 ± 0.3a Phytofluene 7.5 ± 0.2c 9.1 ± 0.1ab 8.7 ± 0.1b 9.5 ± 0.3a Total carotenoids* 119.4 ± 0.6b 137.4 ± 0.7a 113.6 ± 0.7c 104.8 ± 1.1d

Phenol Compounds Chlorogenic acid 46.2 ± 1.5a ND 4.1 ± 0.3b 5.9 ± 0.0b Caffeic acid 19.4 ± 0.1b ND 31.4 ± 3.2a 30.8 ± 2.4a p-Coumaric acid 7.6 ± 0.8c ND 8.8 ± 0.3b 9.9 ± 0.8a Total phenol compounds* 73.2 ± 0.8a ND 44.3 ± 3.2b 46.6 ± 2.0b

Ascorbic acid 33.4 ± 1.5a 29.3 ± 0.5b 20.6 ± 0.3c ND ۞ Values are presented as mean value ± SD (n = 3) and expressed as mg/100 g of dry weight. Different letters in the same row indicate that the values are significantly different (p ≤ 0.05). * The total was obtained by summing each single replicate of each compound. ND = not detected.

Better preservation of α- and β-carotene during boiling compared to steaming was also observed by Kao et al., 2014 and 2012, who concluded that water boiling was the method that determines the greatest stability of these two compounds in carrots compared to water pressure cooking and steaming at 115–120 °C. The authors explained their findings by stating that temperature, instead of the presence of water, was the major factor influencing the carotenoids stability (Kao et al., 2014 and 2012). In the present study, the temperatures during steaming and boiling were the same (100 °C), but steaming of carrots required a longer time in comparison to boiling to reach the appropriate tenderness. Thus, the prolonged exposure to oxygen and light may explain the lower carotenoids recovery observed after steaming than a temperature effect. In the case of frying, major losses may be explained by the lipophilic nature of carotenoids and their instability in the high temperatures reached in this process (i.e., 170-175 °C) as described by Sulaeman et al. (2004).


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During all treatments, similar amounts of carotene cis isomer were formed. The trans–cis- isomerization of β-carotene has been well-documented during carrot processing involving thermal treatments, whereas that of α-carotene has been less investigated (Adriana et al., 2016; Aman et al., 2005).

Phytoene and phytofluene concentrations increased after all three cooking treatments, especially in the case of frying. These two molecules are carotenoid precursors located inside the plant plastids, and their concentration increase may be the result of the release from plastids because of matrix softening during heat processing (Kalt, 2005; Kao et al., 2012).

The predominant phenolic acids of raw carrots were chlorogenic acid, followed by caffeic and p-coumaric acids (Table 2). Boiling had the most detrimental effect on carrot polyphenols, resulting in a complete loss of each compound likely because of their diffusion in the boiling water. Steaming and frying had a less negative effect on total phenolics (-39.5 and -36.3%, respectively), exclusively because of the loss of chlorogenic acid (-91.1 and -87.2% for the two processes, respectively). Phenolic acids are dissolved in vacuoles and apoplast (Kalt, 2005). Cooking of vegetables determines softening and breaking of cellular components with the consequent release of these molecules into the boiling water. The higher softening observed for boiled carrots (Figure 1) well explains the complete loss of polyphenols in comparison to steamed and fried samples.

During steaming and frying, moreover, the hydrolysis of chlorogenic acid into caffeic and quinic acids may also be occurred, justifying the significant increases of caffeic acid observed for both cooking methods (61.9% and 58.8%, respectively). Moreover, polyphenol losses could also be due to the covalent binding between oxidized phenols and proteins or amino acids as well as the polymerization of oxidized phenols (Juániz et al., 2016). These losses could have also in turn affected the color of carrots, especially on the external surface, contributing to the lower a* and higher hue angle (H°) values observed for boiled carrots in comparison to steamed and fried products.

Raw carrots had low value of ascorbic acid (3.9 and 33.4 mg/100 g for fresh and dry matter, respectively) in comparison to other vegetables (Gokmen et al., 2000). The ascorbic acid concentration was slightly but significantly (p ≤ 0.05) affected by boiling (-12.01%) and steaming (-38.14%), whereas it was not detectable in fried carrots. The loss of ascorbic acid can probably be ascribed to water leaching and its thermal degradation, as already reported (Lee and Kader, 2000; Sulaeman et al., 2004).

3. Effect of Cooking on the Total Antioxidant Capacity (TAC):

The TAC values of raw carrots (Table 3) are in agreement with the literature data (Pellegrini et al., 2003). All cooking methods significantly increased carrot TAC, except in the case of steamed carrots measured by the TRAP assay. Frying determined the highest TAC increases, followed by boiling and steaming (Table 3). These results are in agreement with Juániz et al. (2016); Mayer-Miebach et al. (2005), who reported a significant increase of carrot TAC during a thermal treatment at 130 °C for 20 min.

Looking at single antioxidant compounds, boiling determined the highest ascorbic acid and carotenoid retention, a complete loss of polyphenols, and the highest formation of carotene cis isomers. Among these isomers, those of α-carotene have been reported to have higher antioxidant capacity compared to trans counterparts (Adriana et al., 2016; Böhm et al., 2002). The enhancement of carotene availability and their transformation into more active compounds could be both responsible for the TAC increase. On the other hand, steaming induced major losses of ascorbic acid and carotenoids compared to boiling but induced a higher retention of phenolic acids. Frying determined similar carotenoid and polyphenol retentions to steaming but caused a complete loss of ascorbic acid. In this case, the TAC increment is probably also due to the formation of new molecules with high antioxidant capacities, such as Maillard reaction products, because oil absorption and its contribution to TAC were both negligible (Mirzaei et al., 2014).


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Table 3: Effect of different cooking methods on the total antioxidant capacity (TEAC, TRAP, and FRAP values) of raw and cooked selected vegetables۞.

Antioxidant Capacity

Cooking Methods Raw Boiled Steamed Fried

Carrots

TEAC 0.42 ± 0.03d 0.85 ± 0.02b (102%)

0.73 ± 0.04c (74%)

1.02 ± 0.04a

(143%)

FRAP 0.71 ± 0.04d 1.49 ± 0.04b (110%)

1.22 ± 0.05c (72%)

3.19 ± 0.03a (349%)

TRAP 0.04 ± 0.01c 0.25 ± 0.01b (525%)

0.06 ± 0.01c (50%)

0.61 ± 0.01a (1425%)

Zucchini

TEAC 0.82 ± 0.01c 1.55 ± 0.10ab (89%)

1.44 ± 0.06b (76%)

1.67 ± 0.08a

(104%)

FRAP 2.83 ± 0.08c 6.34 ± 0.80b (124%)

5.95 ± 0.07b (110%)

7.94 ± 0.11a

(181%)

TRAP 0.23 ± 0.05d 0.34 ± 0.05c (48%)

0.42 ± 0.02b (83%)

0.82 ± 0.05a (257%)

Broccoli

TEAC 1.13 ± 0.09d 2.20 ± 0.12c (95%)

3.57 ± 0.11a (216%)

2.92 ± 0.21b (158%)

FRAP 5.25 ± 0.51c 8.94 ± 0.54b

(70%) 12.03 ± 0.43a

(129%) 9.11 ± 1.10b

(74%)

TRAP 1.65 ± 0.06c 1.99 ± 0.10c (21%)

3.65 ± 0.33a (121%)

2.45 ± 0.29b (48%)

۞ The percent variation because of cooking is given in parentheses. Values are presented as mean value ± SD (n = 3) and referred to the dry weight. Different letters in the same row indicate that the values are significantly different (p ≤ 0.05). TEAC: (mmol of Trolox/100 g). FRAP: (mmol of Fe+2/100 g). TRAP: (mmol of Trolox/100 g).

Zucchini (Green squash): 1. Effect of Cooking on Physicochemical Characteristics:

Figure 2 shows shear force values that were also obtained for raw and cooked zucchini (Green squash) by three different cooking methods. The Shear force of raw samples was 126.8 ± 4.1 N. Cooking induced significantly higher softening (90.5%) in boiled and steamed products (88.4%) in comparison to frying (84.6%). However, softening of zucchini was lower than that of carrots.

Color determinations were carried out on both external and internal surfaces (Table 4), as was done for carrots. The internal surface of cooked zucchini showed significantly lower coordinates L* and b* values, while coordinate -a* (greenness) did not significantly vary in comparison to raw samples. The external color of the cooked zucchini samples also showed significant changes. Frying induced the highest L* decrement in comparison to raw samples, while coordinates a* and b* were more influenced by the other two cooking methods. In particular, steamed zucchini became less green (higher -a*) and boiled samples became less yellow (lower b*) than the other two treatments, respectively. Chroma color values significantly decreased both externally and internally. The hue angle (HO) significantly decreased on the external surface only for steamed and fried products, resulting in a shift from green to yellow. On the contrary, this color parameter was found to increase internally.

Changes in the visual color observed on the skin of zucchini [loss of chroma (C*) and greenness (-a*)] could be mainly related to the conversion of chlorophyll into pheophytin because of heat treatment, as commonly referred for green vegetables (Gunathilake and Ranaweera, 2016; Turkmen et al., 2006). However, the shift of the hue angle (H°) to yellow was more pronounced for both steamed and fried zucchini than for boiled ones. A more consistent color retention of boiled green vegetables has also been attributed by several authors not only to a different pattern of chlorophyll conversion but also to a change in surface reflectance and depth of light penetration into tissues of boiled vegetables, caused by the loss of air and other dissolved gases by cells and their replacement by cooking water and cell juices (Turkmen et al., 2005).


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Table 4: Effect of different cooking methods on the chromatic coordinates (L*, a*, b*, C*, and H° values) of raw and cooked Zucchini۞.

Cooking Methods

Chromatic Color

L* a* b* C* H° Internal surface Raw 84.9 ± 1.4a -4.7 ± 0.5a 33.5 ± 1.1a 32.6 ± 1.1a 101.6±1.2b

Boiled 54.2 ± 0.7b -4.5 ± 0.3a 16.7 ± 1.0c 16.3 ± 1.0c 109.8±1.4a

Steamed 53.5 ± 0.9b -4.7 ± 0.5a 18.9±1.2bc 18.7 ± 1.1b 108.6±1.3a

Fried 48.8 ± 0.6c -4.9 ± 0.2a 19.9 ± 0.9b 18.4 ± 1.2b 108.8±1.1a

External surface Raw 36.1 ± 1.1a -5.3 ± 0.5c 12.2 ± 1.0a 13.7 ± 1.2a 118.6±1.2a

Boiled 32.4 ± 0.8b -4.0 ± 0.4b 8.1 ± 0.7c 9.3 ± 0.9b 120.4±1.4a

Steamed 33.2 ± 1.2b -2.4 ± 0.4a 11.2±1.1ab 11.6±1.2ab 103.5±1.3c

Fried 28.1 ± 0.9c -3.7 ± 0.6b 9.9 ± 0.9b 11.0±1.2ab 114.7±1.2b ۞ Values are expressed in colorimetric units and presented as mean ± SD (n = 15). Different letters in the same column indicate that the

values are significantly different (p ≤ 0.05).


The content of carotenoids, polyphenols, and ascorbic acid of raw and cooked zucchini (Green squash) are reported on a dry weight basis in Table 5.

To our knowledge, no comprehensive report exists on phytochemical contents of zucchini. In this study, low concentrations of both carotenoids, mostly lutein (2.80 mg/100 g of fresh weight corresponding to 47.1 mg/100 g of dry matter), and polyphenols (3.76 mg/100 g of fresh weight corresponding to 63.2 mg/100 g of dry weight) and a high concentration of ascorbic acid (11.75 mg/100 g of fresh weight and 197.5 mg/100 g of dry weight) were measured in raw zucchini. Small quantities of α-carotene and β-carotene (0.21 and 0.34 mg/100 g of fresh weight corresponding to 3.6 and 5.7 mg/100 g on a dry weight basis, respectively) were observed. Among phenol compounds, caffeic acid had the highest concentration (2.49 mg/100 g of fresh weight and 41.9 mg/100 g on a dry weight basis), whereas similar quantities of chlorogenic and p-coumaric acids (0.64 and 0.63 mg/100 g of fresh weight corresponding to 10.7 and 10.6 mg/100 g on a dry weight basis, respectively) were detected.

Boiling did not affect the total carotenoid concentration (p ≥ 0.05), while steaming and frying resulted in significant losses (18.0% and 31.0%, respectively). As already stated for carrots, losses of carotenoids could be explained by a longer exposure of steamed samples to light and oxygen and by a higher temperature reached during frying, in comparison to boiling (Sulaeman et al., 2004).


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Table 5: Effect of different cooking methods on the antioxidant compounds of raw and cooked Zucchini۞. Antioxidant Compounds Cooking Methods


Llutein 47.1 ± 0.4a 41.4 ± 1.6b 32.6 ± 0.8c 27.9 ± 0.7d

α-Carotene 3.6 ± 0.2b 3.8 ± 0.2ab 4.1 ± 0.1a 3.3 ± 0.2c

β-Carotene 5.7 ± 0.2b 6.4 ± 0.4a 6.5 ± 0.2a 5.2 ± 0.3c

Phytoene ND 1.2 ± 0.1 1.4 ± 0.1 1.2 ± 0.2

Phytofluene ND 1.3 ± 0.1 1.6 ± 0.1 1.4 ± 0.3

Total carotenoids* 56.4 ± 0.2a 54.1 ± 1.3a 46.2 ± 0.9b 38.9 ± 0.8c

Phenol Compounds Chlorogenic acid 10.7 ± 0.1 ND ND ND

Caffeic acid 41.9 ± 0.8a 14.4 ± 0.4d 30.1 ± 1.3b 19.9 ± 1.1c

p-Coumaric acid 10.6 ± 0.2a 5.2 ± 0.3c 9.2 ± 0.4b 5.9 ± 0.3c

Total phenol compounds* 63.2 ± 1.2a 19.6 ± 0.5d 39.3 ± 1.1b 25.8 ± 1.2c

Ascorbic acid 197.5 ± 4.7a 185.7 ± 4.6b 168.8 ± 3.8c 170.1 ± 3.2c ۞ Values are presented as mean value ± SD (n = 3) and expressed as mg/100 g of dry weight. Different letters in the same row indicate that the values are significantly different (p ≤ 0.05). * The total was obtained by summing each single replicate of each compound. ND = not detected.

Boiling and steaming determined an increase of α- and β-carotene but resulted in significant losses (p ≤ 0.05) of lutein (13.2% for boiling and 31.7% for steaming). In addition, both β-carotene and lutein were negatively affected by frying, but minor α- and β-carotene losses (only ∼8.5%) were observed, suggesting a different thermal stability of these compounds. This finding is in agreement with Aman et al. (2005), who studied the thermal stability of chloroplast-bound carotenoids, demonstrating in this system a higher stability of α- and β-carotene than lutein.

The loss of lutein could have also influenced color change (e.g., decrease of L* and b* but increase of the hue angle (H°)) observed not only on the external green skin but also in the yellowish-white internal portion of cooked zucchini. In general, greater losses of zucchini polyphenols were recorded after boiling and frying than after steaming (Table 5). As already observed for carrots, chlorogenic acid was completely lost during all cooking processes, but in the case of zucchini, no caffeic acid increments were recorded. This seems to indicate that losses of phenolic acids occur more quickly in cooked zucchini than in cooked carrots.

Ascorbic acid was not dramatically affected by cooking processes. Steaming and frying had a similar effect on the ascorbic acid concentration, determining losses of about 13%, whereas no significant losses were recorded after boiling (p ≥ 0.05). As already noticed for carrots, boiling had a surprisingly lower detrimental effect on the ascorbic acid concentration than other cooking methods. It is well-known that heat induces a significant loss of ascorbic acid (Kalt, 2005), but this loss was also found to be time-dependent in vegetables (Jiratanan and Liu, 2004). Thus, the lower time of boiling in comparison to steaming could explain the lower losses of ascorbic acid content observed in carrots and zucchini (Adriana et al., 2016).

3. Effect of Cooking on the Total Antioxidant Capacities (TAC):

TAC values for zucchini (Green Squash) are showed in Table 3. Frying determined the highest TAC increases for zucchini, as already observed for carrots, whereas boiling and steaming determined minor increases. During frying, the increases of TAC values were probably due to the formation of Maillard reaction products having antioxidant activities (Jiratanan and Liu, 2004). This formation should also explain the color changes (e.g., decrease of L* and C*) of both external and internal portions of the fried zucchini (Table 2). The enhancement of carotene availability and their transformation into more active compounds could be both responsible for the TAC increase. Moreover, it has been suggested that processing can promote the oxidation of polyphenols to an intermediate oxidation state, which can exhibit a higher radical scavenging efficiency than the non-oxidized ones (Ou et al., 2002; Turkmen et al., 2005).


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Broccoli: 1. Effect of Cooking on Physicochemical Characteristics:

Figure 3 shows shear force values obtained for fresh (raw) and cooked Broccoli. The degree of softening induced by the different cooking treatments as referred to the raw samples was also reported in the same graph. The shear force value of raw broccoli was 93.4 ± 4.2 N. Cooking of fresh broccoli caused a decrease in the force needed to shred the vegetable (Figure 3), indicating a decrease of firmness and consequently softening of the vegetable for all of the three cooking methods. All cooking treatments induced softening, even though lower than that was observed for other vegetables (Carrots and Zucchini), more significantly in boiling (82.3%) and steaming (80.5%) than in frying (76.5%).

Color measurements were obtained both for florets and stems, as presented in Table 6.

Different cooking methods exert a significant effect on the color changes of Broccoli. Coordinate L* values significantly decreased for florets in all cooking treatments. Steamed and fried florets became less green (-a* increase). On the contrary, boiled florets showed a significant increase in greenness (-a* decrease). Coordinates b* and C* values significantly increased for both boiled and steamed florets, while a significant decrease was observed for frying in comparison to the uncooked product (fresh samples). The hue angle (H°) significantly shifted toward yellow for steamed and fried florets. The stem color was characterized by a decrease of coordinates L* and b* values. Greenness (-a*) increased for boiled stems, although not significantly, in comparison to raw broccoli. On the contrary, stems of steamed and fried broccoli samples were significantly less green than raw ones. Coordinate C* significantly decreased only for steamed stems, whereas the hue angle (H°) did not significantly change for boiled stem in comparison to the raw broccoli sample as well as for florets. The color of florets and stems of raw and cooked broccoli samples was comparable to those obtained in a previous study (Mølmann et al., 2015; Munyaka et al., 2010), although broccoli was treated differently prior to cooking.

It is noteworthy that boiled broccoli retained a green color both for florets and stems. The green color intensity of raw and cooked vegetables was reported to be related not only to the pigment concentration but also to light scattering and reflectance of green surfaces (Ramesh, 2000), as reported above for zucchini. An increase of the green color intensity because of a change in surface-reflecting properties, as air between cells was removed and expulsed, was reported for broccoli after the first stage of blanching (Tijskens et al., 2001) or short time treatment (i.e., microwave cooking) in comparison to boiling and steaming (Turkmen et al., 2006). After prolong heating, chlorophyll degradation was reported to cause a decrease in greenness (Koç et al., 2017). Under the same temperature of treatment (100 ⁰C), the shorter time of boiling in comparison to steaming (Mølmann et


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al., 2015) could have partially prevented chlorophyll degradation, inducing only changes in surface-reflecting properties. Table 6: Effect of different cooking methods on the chromatic coordinates (L*, a*, b*, C*, and H° values) of

raw and cooked Broccoli۞ Cooking Methods

Chromatic Color

L* a* b* C* H° Florets Raw 51.6 ± 2.1a -4.1 ± 0.3c 7.4 ± 1.4b 9.5 ± 0.8b 124.4±2.3a

Boiled 37.6 ± 1.2c -6.6 ± 0.9d 12.9 ± 1.9a 15.1 ± 1.4a 124.1±1.8a

Steamed 42.3 ± 2.0b -2.2 ± 1.1b 13.2 ± 2.4a 14.2 ± 1.3a 105.5±1.6b

Fried 33.5 ± 1.2d -0.8 ± 0.2a 3.8 ± 0.8c 4.3 ± 1.1c 101.6±1.3b

Stems Raw 70.3 ± 3.1a -7.9 ± 0.9c 30.7 ± 1.5a 31.4 ± 2.0a 108.4±0.9b

Boiled 57.5 ± 3.0b -8.4 ± 1.1d 24.6 ± 1.4c 27.8±1.8ab 112.5±1.1a

Steamed 53.1 ± 1.9c -6.4 ± 0.9b 25.3±1.6bc 26.3 ± 1.7b 108.4±1.1b

Fried 54.5 ± 1.9bc -5.4 ± 1.0a 28.9±1.5ab 29.8 ± 1.9a 105.5±1.7b ۞ Values are expressed in colorimetric units and presented as mean ± SD (n = 15). Different letters in the same column indicate that the values are significantly different (p ≤ 0.05).


The effects of cooking on carotenoids, polyphenols, and ascorbic acids of broccoli are reported on a dry weight basis in Table 7, in comparison to raw samples. Raw broccoli showed intermediate concentrations of carotenoids, mostly lutein (2.16 mg/100 g of fresh weight and 17.4 mg/100 g on a dry weight basis), and both higher concentrations of polyphenols, mostly chlorogenic acid (4.62 mg/100 g of fresh weight and 37.2mg/100 g on a dry weight basis), and ascorbic acid (106.3 mg/100 g of fresh weight corresponding to 855.7 mg/100 g in terms of dry matter) in comparison to other vegetables analyzed. The flavonols quercetin and kaempferol (2.45 and 2.91 mg/100 of fresh weight, respectively, corresponding to 19.7 and 23.4 mg/100 g of dry weight) were found. These data are in agreement with those recently published in a review on Brassica vegetable antioxidants (Adriana et al., 2016; Podsêdek, 2007).

Cooking of green fresh vegetables has been reported to promote the release of carotenoids from the matrix because of the disruption of carotenoid-protein complexes, leading to better extractability and higher concentrations in cooked samples (Bernhardt and Schlich, 2006; Mølmann et al., 2015). Accordingly, in the present study, the contents of all carotenoid compounds significantly increased in boiled and steamed broccoli (+29.6 and +20.7%, respectively) in comparison to raw ones. The concentration of phytoene and phytofluene almost tripled (Table 7). The release of carotenoids, mainly lutein, from the cells could partially have contributed to the significant increase of the b* value observed for boiled and steamed florets (Table 6) in addition to other phenomena described above. Conversely, frying determined a 62.9% loss of the initial carotenoid concentration, probably because of leaching into oil and to a higher processing temperature (170-175°C). In this case, phytoene and β-cryptoxanthin were completely lost, whereas phytofluene exhibited a similar behavior as in boiling and steaming (Table 7).

As observed for carrots and zucchini, boiling and frying determined a higher loss of total phenolics than steaming. Among phenolic acids, caffeic acid was not affected by the cooking process (p ≥ 0.05), probably because of the hydrolysis of chlorogenic acid that determines a new formation of this compound. The decrease of other phenolic acids could be ascribed to their autoxidation that, in turn, determined the polymerization and/or browning reaction (Jorge et al., 2017). These reactions have probably affected the color of fried broccoli, mainly florets (L* decrease and a* increase) as observed in Table 6. As far as flavonols, quercetin and kaempferol were in general better preserved by steaming than by boiling and frying, in agreement with Vallejo et al. (2003).

The ascorbic acid concentration significantly decreased after all cooking treatments, especially after frying, where a loss of 85.8% was detected. In steamed broccoli samples, a reduction of 31.7% of ascorbic acid with respect to the uncooked sample was observed, in contrast to Munyaka et al.,


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2010, who observed no change of the vitamin C content (both ascorbic and dehydro-ascorbic acids) during steaming.

Table 7: Effect of different cooking methods on the antioxidant compounds of raw and cooked Broccoli۞ .

Antioxidant Compounds Cooking Methods


Llutein 17.4 ± 0.5c 23.0 ± 0.4a 18.7 ± 0.3b 5.6 ± 0.3d

α-Carotene 2.5 ± 0.1b 4.3 ± 0.1a 4.6 ± 0.1a 0.8 ± 0.1c

β-Carotene 6.8 ± 0.2b 7.7 ± 0.2a 7.9 ± 0.2a 3.1 ± 0.2c

β -cryptoxanthin 4.3 ± 0.2a 3.8 ± 0.2b 3.3 ± 0.1b ND

Phytoene 2.4 ± 0.1b 3.9 ± 0.2a 4.1 ± 0.1a ND

Phytofluene 2.2 ± 0.1c 3.7 ± 0.1b 4.6 ± 0.2a 3.8 ± 0.1b

Total carotenoids* 35.8 ± 0.4c 46.4 ± 0.4a 43.2 ± 0.6b 13.3 ± 0.3d

Phenol Compounds Chlorogenic acid 37.2 ± 2.1a 14.1 ± 1.3bc 16.3 ± 1.4b 11.4 ± 0.9c

Caffeic acid 4.1 ± 0.8a 4.6 ± 0.5a 4.3 ± 0.7a 4.7 ± 0.5a

Sinapic acid 8.6 ± 1.1a 3.5 ± 0.6c 4.7 ± 0.6b 4.9 ± 0.6b

Ferulic acid 9.8 ± 1.2a 2.2 ± 0.6c 3.5 ± 0.5b 4.1 ± 1.5b

Quercetin 19.7 ± 1.7a 3.1 ± 0.8d 12.6 ± 1.2b 6.1 ± 0.7c

Kaempferol 23.4 ± 1.9a 3.3 ± 0.2c 23.3 ± 1.7a 12.7 ± 0.9b

Total phenol compounds* 102.8 ± 3.2a 30.8 ± 1.4d 64.7 ± 2.3b 41.5 ± 1.7c

Ascorbic acid 855.7 ± 4.4a 445.3 ± 3.4c 584.8 ± 4.5b 121.9 ± 2.2d ۞ Values are presented as mean value ± SD (n = 3) and expressed as mg/100 g of dry weight. Different letters in the same row indicate that the values are significantly different (p ≤ 0.05). * The total was obtained by summing each single replicate of each compound. ND = not detected.

3. Effect of Cooking on the Total Antioxidant Capacity (TAC):

The TAC values of raw broccoli samples (Table 3) were lower than those of a previous study (Pellegrini et al., 2000). This is likely related to the high variability of this parameter observed in the Brassica family (Ou et al., 2002). Steaming determined the higher TAC increase probably because of the increase of carotenoids and the major retention of polyphenols and ascorbic acid with respect to the other two methods (boiling and frying). These findings are in agreement with Glyszczynska-S� wiglo et al. (2006), who observed an increase of TEAC values of ascorbic acid, polyphenol, and carotenoid extracts in steamed broccoli samples.

4. Effect of Cooking on the Glucosinolate Compounds of Broccoli:

Beside antioxidant compounds, vegetables belonging to the Brassica family, including broccoli, are characterized by the presence of glucosinolates compounds that together with their breakdown products are considered biologically active and have cancer-protective effects (Mølmann et al., 2015). Thus, the glucosinolate content of raw and cooked broccoli was also determined and is presented in Table 8 and Figure 4. In agreement with data by Jorge et al. (2017), the predominant glucosinolates of raw broccoli were neoglucobrassicin (1-methoxy-glucobrassicin), which was present in a very high concentration, and glucobrassicin, followed by glucoraphanin, the only aliphatic compounds identified.

Data on the cooking effect showed that total glucosinolate concentrations were significantly modified by the cooking treatment (Figure 4). Steaming was the only cooking method that completely preserved glucosinolates and even significantly increased by 19% of their initial concentration (p ≤ 0.05). On the contrary, boiling and frying caused a substantial degradation of these molecules, especially evident in the case of frying that determined a total loss of 80%.


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Table 8: Effect of different cooking methods on the Glucosinolate compounds of raw and cooked Broccoli*.

Cooking Methods

Glucosinolate Compounds Glucoraphanin Glucobrassicin 4-methoxy-

glucobrassicin 1-methoxy-

glucobrassicin Total

Glucosinolates**

Raw 2.2 ± 0.8b 11. 6 ± 1.1ab 1.7 ± 0.8bc 59.8 ± 3.1b 75.3 ± 3.1b

Boiled 2.9 ± 0.9a 9.5 ± 0.9b 1.9 ± 0.8b 18.6 ± 2.1c 32.9 ± 1.6c

Steamed 1.9 ± 0.8bc 15.4 ± 1.8a 3.1 ± 1.0a 69.3 ± 3.7a 89.7 ± 3.5a

Fried 1.6 ± 0.9c 2.7 ± 0.9c 1.5 ± 0.7c 9.6 ± 0.9d 15.4 ± 1.1d * Values are presented as mean value ± SD (n = 3) and expressed as μmol/g of dry weight. Different letters in the same column indicate that the values are significantly different (p ≤ 0.05). ** The total was obtained by summing each single replicate of each compound.

Glucosinolates are water-soluble compounds and are usually lost during conventional cooking because of leaching into surrounding water (Mølmann et al., 2015). Moreover, degradation events at higher temperature, such as during frying, may also occur, leading to the formation of volatile compounds that were not determined in this study. On the contrary, steaming was reported to well preserve (Vallejo et al., 2002) or increase (Glyszczynska-S� wiglo et al., 2006) broccoli glucosinolates. The glucosinolate increase could be partially explained not only by the inactivation of myrosinase, as suggested by Vallejo et al. (2002), but also by a disintegration of plant tissue upon heat because part of these molecules are bound to the cell walls and released only after a disintegration of cell structures (Glyszczynska-S� wiglo et al., 2006).

Among single glucosinolates, glucoraphanin was the compound for which a lower decrease during frying was observed, confirming the higher thermostability of aliphatic glucosinolates than indolyl ones (Munyaka et al., 2010; Vallejo et al., 2002). The enzymatic hydrolysis of glucoraphanin produces the isothiocyanate sulforaphane that seems to be involved in the reduced incidence of a number of tumors in both in Vitro and in Vivo studies (Jorge et al., 2017; Link et al., 2004). Thus, the retention of glucoraphanin during processing may be of great biological interest, because its breakdown into bioactive compounds has been reported to occur also within the intestinal tract by microflora (Murador et al., 2016; Holst and Williamson, 2004). Conclusion

The present study clearly indicates that physicochemical characteristics, phytochemical profile, antioxidant capacity and color parameters of selected vegetables (carrots, are deeply modified by domestic cooking and that modification of the evaluated parameters are also strongly dependent upon the vegetable species. However, the cooking conditions used here, chosen to reproduce the common Egyptian cooking practices (boiling, steaming and frying) were less severe than thermal conditions usually applied in previous studies. These conditions would have promoted the release of antioxidant compounds from the vegetable matrix and determined the formation of new antioxidant compounds.


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Moreover, it is also likely that matrix softening and increased extractability upon cooking were accompanied by the conversion of polyphenol into very active chemical species, which were not yet identified and concurred synergistically to determine the high antioxidant capacity.

The overall increase of TAC values observed in the present study is in partial disagreement with the concept that processed vegetables have lower nutritional quality than the raw (fresh) ones. Moreover, our results suggest that for each vegetable a preferential cooking method could be selected to preserve or improve its physicochemical characteristics and antioxidant properties. This selection may help consumers on the choice of cooking practices to improve the nutritional quality of foods, as well as their acceptability. In this investigation, the three vegetables did not demonstrate a consistent trend under the same cooking conditions, which could be ascribed to morphological features, specific components, and antioxidants profile of vegetables.

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Influence of Different Cooking Methods on Physicochemical ...curresweb.com/mejas/mejas/2017/1127-1147.pdf · Received: 18 Sept. 2017 / Accepted: 22 Nov. 2017 / Publication date: 30